Science Fiction is an adventure, mystery, drama (heck, any genere) that has some basis in a scientific fact or wild theory (faster that light space travel is a great example). The level of research, or reality towards the science determines what kind of science fiction it is. For example, Star Wars uses a few facts (or crazy theories if you prefer), such as the idea of FTL travel through "hyperspace"; but since it's all pretty improbable, star wars type stuff is considered "space opera". Just unplug your analytical mind and watch the pretty lightsabers.

If the science is sound and well-researched, like some of Ben Bova's (one example, there are many other authors) writing, than it's considered "hard" science fiction and calls on a broad knowledge base of popular science. Knowing a little bit about the physics or chemistry (or history sometimes) helps you appreciate and enjoy this kind of science fiction.

for instance, in medieval times, people who saw strange lights or experienced other strange phenonema described the experiences in terms of meeting angels or pixies, whereas, nowadays, thanks to science fiction, people are as likely to interpret these experiences as alien visitations, abductions, or ufo sightings.

An elaborate, suspenseful, and sometimes well-written fairy tale that typically focuses on paranormal events, aliens, technology, futuristic visions, space exploration, multiple dimensions, genetics, and other themes which require a good use of logic and scientific theories/facts.

Thank God for the Sci Fi channel. Now I can tape all my favorite episodes of the Outer Limits. :^))

A genre of naturalistic fantasy that usually has no basis whatsoever in scientific fact.

'Star Wars' is not science fiction; it's a big dumb fantasy movie about space wizards. 'The Matrix' is not science ficiton either; it's a comic book action movie with a pretentious pseudo-philosophy thrown in. What then, constitutes science fiction? A piece that actually cares about the plausibility of its ideas; a rarified sub-set of literature and cinema to be certain.